AtomicInteger 源码分析

private static final Unsafe unsafe = Unsafe.getUnsafe();
   // value字段的偏移量
    private static final long valueOffset;

    static {
        try {
           //获取value字段的偏移量
            valueOffset = unsafe.objectFieldOffset
                (AtomicInteger.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }
   //volatile修饰value从而保证value值的线程可见性
    private volatile int value;

    public AtomicInteger(int initialValue) {
        value = initialValue;
    }

 public final int incrementAndGet() {
        return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
 }

//Unsafe源码
  public final int getAndAddInt(Object var1, long var2, int var4) {
    int var5;
    do {
      //通过字段的内存地址获取最新的值
      var5 = this.getIntVolatile(var1, var2);
                   //CAS更新新值,如果失败重试
    } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
    return var5;
  }

var1,var2联合定位字段的内存地址

compareAndSwapInt 是CAS操作更新新值,当compareAndSwapInt失败,do代码块会再次执行,获取最新的值var5,然后重新执行compareAndSwapInt,直至成功为止.